Preface

Most of the GNU Emacs integrated environment is written in the programming
language called Emacs Lisp. The code written in this programming
language is the software—the sets of instructions—that tell the
computer what to do when you give it commands. Emacs is designed so
that you can write new code in Emacs Lisp and easily install it as an
extension to the editor.

(GNU Emacs is sometimes called an “extensible editor”, but it does
much more than provide editing capabilities. It is better to refer to
Emacs as an “extensible computing environment”. However, that
phrase is quite a mouthful. It is easier to refer to Emacs simply as
an editor. Moreover, everything you do in Emacs—find the Mayan date
and phases of the moon, simplify polynomials, debug code, manage
files, read letters, write books—all these activities are kinds of
editing in the most general sense of the word.)